You don't want to miss extraction! At the start of Wave 11, head to the extraction zone, stay there and do everything you can to keep it clear from enemies.
Try to conserve your missiles during objective waves so that you can use them in the beginning of Wave 11 to make the extraction a little easier for everyone.
Snipers are powerful characters but weak at close-range combat. Use powerful close-range abilities such as Multi-Frag Grenades, Flamer, Shockwave, Smash etc. liberally around your sniper teammates so that they can focus on what they do best.
Biotics are strong against armor but weak against shields, which naturally makes Disruptor Ammo the best choice for all biotic kits. If you have biotic teammates, you should try to help them by spraying your Disruptor Ammo on the targets they're trying to use their biotics on.
Power Recharge Speed is, perphaps, the most important statistic of your character, as it determines how effective you can be with your powers. If your recharge speed drops below 200%, you're severely gimping your character and putting your team at risk.